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Digital Subscriber Line - ADSL, SDSL, VDSL, DSL, IDSL and ISDN

There are different types of Digital Subscriber Lines, all of which use digital signals over analog telephone wires. This translates to higher data speeds than what a typical analog 56Kbps modem is capable of since it no longer needs to convert digital to analog and vice versa. Here are the most common types of DSL:

• ADSL – Asymmetric Digital Subscriber Line is an asymmetric service, which means the download speeds are faster than the upload speeds. This service is ideal for single users with high-speed data needs. It is an excellent solution for home Web users who would like to conduct quick, efficient Web surfing, Internet research, large file downloads, video streaming, audio streaming, and 3-D image posting. ADSL operates at speeds of 384/128 Kbps or 768/384 Kbps, depending on a user's distance from their telephone company's central office.
  ADSL is an asymmetrical service that is most commonly offered by telephone companies. ADSL is not suited for business uses such as hosting Web or e-mail servers. In addition, ADSL often uses a bridge instead of a router that provides little to no security, making your network vulnerable to attacks. Finally, telephone companies generally don't offer service level agreements on their ADSL service because their target customers are residential.
• IDSL – ISDN Digital Subscriber Line is a symmetric service that can deliver high-speed data communication up to 128 Kbps. IDSL is an ideal high-speed service alternative for users living or working outside of the metropolitan periphery, beyond the reach of traditional SDSL and ADSL services. In many non-urban areas, the distance between a telephone company's central office and a user's business or home can be substantial. IDSL's strongest asset is its ability to reach up to 36,000 feet from the telephone central office. This is twice the distance of SDSL.
• SDSL – Symmetric Digital Subscriber Line is a symmetric service, and one of the first iterations of broadband – which means that the upload data throughput from your computer to the internet is the same as your download speed. SDSL can deliver high-speed data communications at up to 1.54 Mbps. This service is delivered via a single pair of copper wires and is ideal for business applications that would have otherwise been served by an expensive T1. SDSL is primarily geared for web hosting and teleconferencing.
• VDSL - Another developing broadband technology is Very High Data Rate DSL, or VDSL. VDSL promises to offer speeds of up to 50Mbps for lines up to 300 metres in length and runs on existing single copper pairs. There are two competing standards of VDSL: one uses the same technology as ADSL and essentially adds extra frequencies to the standard ADSL frequencies; and a second which employs a completely different coding scheme.
  Unfortunately, VDSL is a very expensive technology to deliver and is unlikely to be deployed on a wide scale because most customers won't be close enough to the telephone exchange to be practical.
  While VDSL has a much higher possible throughput than ADSL, it doesn't really help in extending reach over ADSL. Where it is likely to be used is in an environment where customers are close to the termination point and a service such as video requiring more than 7Mpbs to be delivered.

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